mildred avatar image
mildred asked

AGM batteries bulging after overcharge from MPPT 150/35, why?


Recently I recabled all my solar system (I moved it somewhere else) and today I was looking at the system and the batteries were deeply bulged, hot on touch, and I would have expected them to explode.

The battery is a 48V 220Ah Victron AGM charged by two 150/35 MPPT chargers.

The current state of charge is 100% and the battery voltage is 52V once I cut all the breakers.

I expect the batteries to be dead now, but I need to know what caused this to avoid repeating the same mistake twice. Today is a sunny day and I expect that the MTTP overcharged the batteries. What I don't understand is why.

Could it be the battery wiring that induced a small resistance causing a voltage drop and causing the MPPT to think the battery voltage to be lower than actual?

There is also something fishy about the circuit breaker of the batteries. I still read just above 1V after the breaker when the breaker is on the OFF position. But I disconnected it and checked the resistance and it seemed to work correctly, also when ON, the breaker does not cause a voltage drop or anything. Perhaps there is a device on the other side of the breaker, a small capacity that causes this...


Also, now, the batteries are arranged a bit differently and there is less space around them. Could it be that they went too hot in their new environment and did not cope with that ? I arranged insulation around them but perhaps the insulated box is now too small and head dissipation could not happen correctly, is that enough to cause this kind of damage?

AGM Battery
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Impressive failure.

Terrible photo angle / quality.

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2 Answers
wkirby avatar image
wkirby answered ·

Charge Voltage limits need to be reduced as lead acid batteries warm up and increased when the battery is cold. This is what the temperature compensation mechanism does. It reduces the charge Voltage by a set ammount per degree increase. The base temperature is 25ºC.

When the battery warms up and the charge Voltage is not reduced then the battery will warm its self up and exacerbate the problem. This is known as thermal runaway.

Without a temperature sensor on the battery then the charge controller takes an ambient temperature measurement at the beginning of the day. This is a good time where the batteries temperature and the MPPT temperature are pretty much the same.
If the batteries are thermally insulated from ambient and are most likely at a different temperature from the MPPT then there will be an error. The MPPT would likely measure a lower ambient temperature from the actual battery which is much cosier in its insulated box. This would cause the temperature compensated charge Voltages to be higher than they should be.

In an installation where the battery temperature could be quite different from the MPPT temperature first thing in the morning, a battery temperature sensor is essential to prevent the problem that you have observed.

If your MPPT's are the Smart type (with Bluetooth) then you would need to use the battery temperature sensor with a Smartshunt or BMV-712. If you don't have one of these battery monitors then the Smart Battery Sense would be able convey actual battery Voltage and temperature to Smart MPPTs and the temperature compensation mechanism would work properly.

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2 x 35a chargers on a 220ah battery is probably a bit above recommended charge current also.
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I have to check but I believe when I designed the system, the panels were not able to produce the max power the MPPT were able to handle. And I had a few summers that went ok that way. I believe it's the new battery enclosure that was too tight.
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Thank you for the explanation, that's probably what happened.

Unfortunately, the MPPT is not the smart version. Is there a device that I could plug in to disconnect the batteries in the event of an abnormal temperature, charge current or voltage ? ideally a protection switch connected to a GX device.

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Ideally you would limit the charging current of the two MPPT chargers to like 22Ah (10% of your 220Ah battery).

This can be done from within VictronConnect in the battery settings of your MPPTs:


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@mildred that's 22 Amps total, so 11 Amps on each.
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pwfarnell avatar image pwfarnell kevgermany ♦♦ ·

From the datasheet for Victron deep cycle AGM batteries

15. Charge current

The charge current should preferably not exceed 0,2C (20A for a 100Ah battery).The temperature of a battery will increase by more than 10°C if the charge current exceeds 0,2C. Therefore temperature compensation is required if the charge current exceeds 0,2C.

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nickdb avatar image
nickdb answered ·

Have you got a GX, does it report to VRM? This would be a good source of information and alerts as to what happened.

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Unfortunately, no, I only moved the system recently and I did not took time yet to install the GX device.
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Without a GX, did you configure the mppt's to share data?

You really would have wanted the charging coordinated between the devices with DVCC, and appropriate limits set.

Without this you can easily run into problems.

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